Currently, the cathode materials for lithium ion secondary batteries are mainly lithium transition metal oxides, including hexagonal layered structure materials such as LiCoO2, LiNiO2, LiNi1−x−yCoxMnyO2 and spinel structure materials such as LiMn2O4. The cathode materials also include polyanion cathode materials such as LiFePO4 with olivine structure. Since the crystal structure of LiNiO2 and LiCoO2 is the same, the theoretical specific capacity would be the same. However, the actual specific capacity of LiNiO2 is larger than 200 mAh/g, which is much higher than that of LiCoO2. In fact, the reserve of nickel (Ni) is higher than cobalt (Co), the price of nickel is lower than cobalt, and the toxicity of nickel is lower than cobalt. Therefore, LiNiO2 is expected to replace LiCoO2 as a preferred cathode material for lithium ion secondary battery.
However, due to the difficulty in producing LiNiO2 and the poor cycling stability and thermal stability of LiNiO2, LiNiO2 has got few commercial applications. When elements such as Co and Al are doped into LiNiO2, these elements can stabilize the crystal structure of LiNiO2, thereby improving the cycling performance and the thermal stability of LiNiO2, reducing the difficulty of producing, and promoting the transition from Ni2+ to Ni3+ in a sintering process. Compared to LiNiO2, Al- and Co-doped LiNiO2 (Li1+δ[Ni1−x−yCoxAly]O2) can significantly improve the performance of the cathode material while maintaining a high capacity. The doped LiNiO2 can be industrially produced in large-scale and is successfully applied as digital power supply and electric vehicle power supply.
Compared to LiNi1−x−yCoxMnyO2 and LiFePO4, the cycling stability and thermal stability of the doped Li1+δ[Ni1−x−yCoxAly]O2 still requires improvement. When the battery using the cathode material of Li1+δ[Ni1−x−yCoxAly]O2 is charged, Ni4+ with high valence and high reactivity will be generated. Ni4+ will react with the electrolyte to generate gas and form an organic polymer on the surface of the cathode material. However, the organic polymer is an electrical insulator and is non-conductive for lithium ions, thereby causing flatulence of the battery and rise of an inner resistance in the positive electrode of the battery. At high temperature, the reaction between Ni4+ and the electrolyte will increase, producing more gas and more organic polymer on the surface of the cathode material. Thus, the surface of the cathode material is deposited continuously with nickel monoxide, which is an insulator with no electrochemical activity, thereby causing the inner resistance in the positive electrode to rise further. Meanwhile, Ni4+ is continuously reduced to Ni2+, causing the reversible capacity of the battery to lose gradually, eventually leading the battery life to an end in a short time. Moreover, due to high reactivity of Ni4+ and the oxygen evolution at high temperature, the battery will probably reach an out-of-control state at a relatively low temperature, causing a security risk in use of the battery.
Surface coating is one of important means to modify and improve the electrochemical properties of the cathode material. Coating materials commonly used are ZnO, ZrO2, AlPO4, Li3PO4, Al2O3, AlF3, SiO2, TiO2, MgO and Li2O-2B2O3, as well as organic polymer such as polyaniline. Although the modification mechanism is still not fully understood, many studies have shown that a number of coating materials can improve the performance of the cathode material more or less, especially the cycle life and the thermal stability of the cathode material. The modification mechanism may be that the coating material works in two aspects. One aspect is the physical isolation, although the coating material cannot cover all the surface of the particle, the coating material can, to a certain extent, reduce the contact between the cathode material and the electrolyte. Another aspect is the reaction with the decomposition product of Lewis acid of the electrolyte, this will reduce the acidity of the interface between the cathode material and the electrolyte to protect the positive electrode. However, the conduction capability of the coating material for conducting lithium ions or electrons is often not ideal, and at least the diffusion coefficient for lithium ions and the conductivity for electrons are lower than the active cathode material. Thus, the coating material on the surface of the cathode material, particularly when coated with an excessive amount, will inevitably cause decline of the rate performance of the cathode material.
Korean patent application No. 1020040118280 and US patent application publication No. 20080160410A1 disclose a core-shell structure in particles to stabilize the nickel-rich cathode material. The core of the particle is formed of a nickel-rich material having high capacity, such as, LiNi0.8Co0.2O2 or LiNi0.8Co0.1Mn0.1O2. The shell of the particle is formed of a material having good thermal stability and long cycling life, such as, LiNi0.5Mn0.5O2. The core provides the high capacity for the cathode material. The shell has a stabilized structure and is not reactive with the electrolyte. The shell provides a physical isolation of the core from the electrolyte and serves as a protection layer for preventing the reaction of the core with the electrolyte, thereby retaining high capacity as well as good thermal stability and long cycling life for the cathode material.
U.S. Pat. No. 8,926,860 and US patent application publication No. 20140131616A1 disclose a cathode material with concentration gradient in whole particle. The metal making up the cathode material has continuous concentration gradient in the entire region, from the center part to the surface part of the particle. For example, the high active metal such as Ni has a highest concentration in the center part and a lowest concentration in the surface part, i.e., the concentration of Ni is gradually decreased from the center part to the surface part. On the contrary, the inactive metal such as Mn has a lowest concentration in the center part and a highest concentration in the surface part, i.e., the concentration of Mn is gradually increased from the center part to the surface part. Other materials such as Co can be distributed uniformly in the particle, or otherwise distributed with continuous concentration gradient throughout the particle. The nickel-rich center part of the particle causes the cathode material having high capacity performance, while the manganese-rich surface part of the particle causes the cathode material having good thermal stability and long cycling life.
From the above core-shell structure or concentration gradient structure of the particles used to form the cathode material, the core idea is using the stable coating material in the surface part to protect the inner active material in the center part, to thereby obtain nickel-rich cathode material having high capacity, good thermal stability and long cycling life. The coating material in the surface part is preferred to be conductive to lithium ions or electrons, to avoid a decrease of capacity, a rise of inner resistance and an influence of rate performance to the cathode material. However, the coating material in the surface part for protection purpose is electrochemically active. The coating material has relatively high charging-discharging specific capacity at a voltage of 4.2V versus Li/Li+. That is, when the battery cell is charged to 4.2V, active elements in the coating material such as Ni, Co, Mn will be oxidized as lithium ions deintercalate. As a result, the surface of the coating material will form active sites, which are electrochemically active. Thus, using outer relatively stable material to protect inner active material still cannot avoid the coating material from reacting with the electrolyte. Furthermore, due to structural difference, the expansions of the outer coating material and the inner core material during charging and discharging are also different. Especially for the core-shell structure, after charging and discharging in repeated times, the shell will separate from the core, thereby causing an interruption of a conduction path inside the particle for conducting lithium ions or electrons.